Conventionally, as one such control device for the internal combustion engine, a device described in Patent Document 1, for example, is known. A typical conventional control device, including the device described in Patent Document 1, opens and closes an EGR valve in a forcible manner when fuel cutoff control is being carried out and determines the pressure change amount ΔP in the intake passage caused by opening or closing the EGR valve. The control device diagnoses whether an abnormality has occurred in the EGR device based on the pressure change amount LP. Specifically, the greater the degree of clogging of the EGR passage, the smaller the pressure change amount ΔP becomes. Accordingly, when the pressure change amount ΔP is lower than or equal to a predetermined value, it is determined that an abnormality has occurred in the EGR device.
During the fuel cutoff control, when the engine speed falls below a predetermined resumption engine speed, the above-described control device for the engine suspends the fuel cutoff control and resumes fuel supply to the engine.
In some cases, even after the engine speed falls below the resumption engine speed, diagnosis as to whether the EGR device has an abnormality may remain incomplete and the EGR valve is maintained open. In these cases, exhaust gas is introduced into the intake passage through the EGR passage, which may cause poor combustion performance in the engine and excessive decrease in the engine speed.
To solve these problems, in conventional techniques, a predetermined forcible suspension engine speed, which is an engine speed higher than the resumption engine speed, is set. When the engine speed falls below the forcible suspension engine speed while the aforementioned diagnosis is being performed, the EGR valve is forcibly closed and the diagnosis is suspended. Further, since the resumption engine speed is variably set, the forcible suspension engine speed is set to a value greater than or equal to the maximum value of the resumption engine speed to prevent excessive decrease of the engine speed even when the resumption engine speed is set to its maximum value. This prevents degradation of the combustion performance of the engine caused by the EGR valve that is held open after the fuel cutoff control is suspended.